1. Field of the Invention
The present invention relates to a fixing apparatus which is used in an image-forming apparatus such as an electrophotographic or electrostatic-recording copying machine or printer and which fixes a toner image.
2. Related Background Art
As for an image-forming apparatus such as an electrophotographic color copying machine, active product development has heretofore been under way. As a toner used in the image-forming apparatus, a non-magnetic toner formed of a material having a lower melting point and lower melt viscosity than those of a monochrome toner is used for the purpose of improving a color-mixing property and the transparency of OHT.
Therefore, at the time of fixing a color toner image formed on a sheet of paper used as a recording material, a so-called xe2x80x9coffsetxe2x80x9d which is the fusion of toner on the surface of a fixing roller which is the fixing member of a fixing apparatus is liable to occur. Thus, a heat-resistant silicone oil is applied as a release agent to the surface of the fixing roller in a fixing apparatus to be installed in an image-forming apparatus such as a color copying machine to prevent the offset of the toner.
Meanwhile, the fixing roller must be provided with moderate elasticity for the purpose of improving the follow-up property and fixation property to a multicolored superposed image, and it has a surface layer composed of silicone rubber having high releasability such as LTV or RTV formed on an elastic layer composed of HTV rubber or the like. This silicone rubber has a high affinity for a silicone oil because the silicone rubber is the same type of material as the silicone oil. The oil permeates the rubber through its surface to impart high releasability to the rubber and to exhibit the effect of preventing the offset.
However, the fixing apparatus provided in the conventional image-forming apparatus has the following problem.
While the fixing roller of the fixing apparatus is directly coated with the oil, a pressing roller which is pressed against and located underneath the fixing roller is often provided with the oil indirectly from the fixing roller via a nip, and the oil can be applied only at a sheet interval at the time of continuous printing. Therefore, the application of the oil to the pressing roller is liable to be intermittent, and the prevention of the offset on the pressing roller is not satisfactory accordingly. Consequently, there is the possibility that there may occur such a problem that the back of sheet is soiled by the toner stuck on the pressing roller for some reason.
The offset on the pressing roller can be prevented by applying the oil to the pressing roller directly. However, since this requires a special application mechanism, it causes such a problem that the apparatus becomes large and complicated.
It is also considered to increase the amount of the oil to be applied to the fixing roller. This is liable to cause an oil stain on a sheet, thereby degrading the quality of an image and the reliability of fixation. Further, the swelling of the fixing roller by the oil is liable to cause a change in the diameter of the roller or peeling of the rubber. To prevent these, for example, an intermediate layer having an oil-barrier function is further required, thereby complicating the constitution of the fixing roller and increasing the costs.
On the other hand, when the contact pressure of a cleaning blade is set to be small by, for example, making the entering amount of the cleaning blade to the pressing roller small so as to secure a certain amount of the oil stuck to the surface of the pressing roller during the passing of the sheets, the passing-through of the oil is liable to occur, whereby an oil blotch on a sheet may occur as in the above case.
Under the above circumstances, it has been recently found that by using toner which contains wax comprising a paraffin or a polyolefin to attain an oil-less fixing apparatus which is capable of fixing a color toner image formed on a sheet, good fixation of a color image can still be accomplished without applying a silicone oil to the fixing roller as has been conventionally practiced.
However, as the fixing roller in a fixing apparatus for fixing a color image, a so-called xe2x80x9csoft rollerxe2x80x9d which is obtained by laminating an elastic layer comprising silicone rubber or fluororesin rubber around a core bar is used. The soft roller is also used in the currently practiced oil-less fixation.
When the soft roller is used at high temperatures of 180 to 200xc2x0 C., the rubber coating layer deteriorates and is therefore liable to peel from the core bar. Thus, it has poor durability as compared with a hard roller having no elastic layer. Further, the soft roller also has the problem that the performance of the fixing roller changes between the initial stage of its use and after undergone some endurance because the hardness of the rubber changes as the rubber deteriorates. In addition, on activation of the copying machine, the fixing roller must be warmed up from room temperature to temperatures of about 160 to 200xc2x0 C. at which toner can be fixed. The time during which the roller is warmed up is called xe2x80x9cwarm-up timexe2x80x9d. The shorter the time is, the more convenient it is for the user of the copying machine. However, the soft roller used in the fixing apparatus for fixing a color image also has the inconvenience that the presence of the elastic layer increases heat capacity and prolongs the warm-up time.
It is an object of the present invention to provide a fixing apparatus which is free of a mechanism for applying a releasing agent to a fixing roller.
It is another object of the present invention to provide a fixing apparatus which provides an unfixed image formed of wax-containing toner by a hard roller.
It is still another object of the present invention to provide a fixing apparatus that comprises a heat-fixing member, said heat-fixing member having a core metal and a surface releasing layer which is provided on said core metal and does not have a rubber elasticity and a pressing member which forms a nip which holds and conveys the recording material holding an unfixed image formed of wax-containing toner, wherein the unfixed image is fixed to the recording material with heat and pressure, and the following expression is satisfied:
2.0xc3x97105xe2x89xa6F/Sxe2x89xa610.0xc3x97105 
wherein F is the pressure-contact force (N) between the fixing member and the pressing member, and S is the area (m2) of the nip.
Other objects of the present invention will be apparent from the following description.